Processes and devices for the contactess electronic control of the flow of water in washing units have been known from, e.g., CH-A-651 143, CH-A-646 766 and DE-A 39 20 581. If these devices are operated from a battery, the battery in time must be replaced upon reaching a certain residual capacity. To indicate that a battery change is due, it was previously determined whether the battery voltage exceeded a predetermined voltage value. An insufficient voltage was indicated by means of an LED.